L. V. Pirsson — Geology of Conanicut Island. . 377 



chemical composition of a specimen from Potter's Point is 

 given in the above analysis. Lime was carefully tested for 

 but not even a trace precipitated. It seems to be a very pure 

 and typical phyllite. Even in the clearest and most uniform 

 examples the original bedding shows in slight changes of a 

 color banding across the cleavage planes. 



Contact metamorphic rocks. — From what has already been 

 said in the foregoing it is evident that no safe petrographical 

 conclusions can be drawn concerning the nature of the meta- 

 morphism which has been effected in the sedimentaries by the 

 intrusion of the granite. For the original strata varied rapidly 

 in thickness and in character ; their exact stratigraphical posi- 

 tion in relation to the granite can no longer be made out, they 

 have suffered from dynamic forces and from weathering as 

 well, each bringing its own series of changes. The original 

 character therefore is strongly masked and can only at times 

 be suspected. Their geological relations are indeed far better 

 seen and studied in the field than with the microscope though 

 the latter reveals some interesting facts. 



The first stage of alteration, where the shales assume a knot- 

 ted appearance, has been sufficiently described, what follows 

 relates to the harder and tougher hornstones. The dark solid 

 slate-like rock occurring on the shore in a recent cutting in the 

 lower part of E 6 shows an extremely fine grain consisting of 

 quartz, white mica, specks of graphitic matter and iron ore 

 with groups composed of masses of fine crystals that appear 

 to be epidote and zoisite. These are strung along in a schis- 

 tose structure. All through it appear places with less highly 

 doubly refracting material that between crossed nicols show 

 idiomorphic outlines and seem like pseudomorphs or phantom 

 shapes of some former mineral that appeared in porphyritic 

 crystals. 



A very common form of the hornstone rocks is one com- 

 posed chiefly of quartz and chlorite together with white mica, 

 iron ore, etc., in allotriomorphic structure. This is assumed 

 to be a rock which was formerly composed chiefly of quartz 

 and biotite. It generally occurs pretty near the granite and 

 composes largely the outcrops of E 4, lower E 5 and to the west 

 of the road in D 5. It occurs also in direct contact with the 

 granite in F 4 on Bull's Point. 



Another form is that which occurs in the outcrop south of 

 Mr. Green's barn in D 5 and east of the road. This as re- 

 marked before begins with a rock which has not entirely lost 

 its shaly character. The rock consists essentially of quartz, 

 white mica, fragments of feldspar, chlorite, bits of graphite and 

 iron ore being freely sprinkled through it. Sections cut from 

 the rock to the south of this show in addition a development 



